1. Field of the Invention
The present invention relates to an exposure mask, a projection exposure substrate for the exposure mask used in a lithography process of a semiconductor device, a method for manufacturing the mask and substrate, and a method for forming a pattern with use of the exposure mask.
2. Description of the Related Art
As semiconductor technology advances, operation speed and integration of a semiconductor device and thus semiconductor elements have been increased and enhanced. Accordingly, the necessity of making a lithographic pattern smaller has been increasingly demanded and thus pattern dimensions have been required to be made smaller with a higher accuracy. For the purpose of satisfying such demand, a light source using such short wave light as transmissive ultraviolet light has been employed as a stepper. In the process of a KrF excimer laser based on a 248 nm oscillation beam, which is expected as a stepper light source in the next generation; chemically-photosensitized resist has been developed as a resist material. However, this process is still in its experimental stage and thus it is difficult at the current stage to put it to practical use. In this way, when the wavelength of a stepper light source is changed, its development must be started from material development, which requires a considerable term until it can be put into practical use.
There have been proposed attempts to make a pattern smaller without changing its exposure light source (stepper). These attempts include, for example, a phase shift method. In this method, a phase inversion layer is partially provided to a light permeable part so that the influence of diffraction of light from an adjacent pattern is removed to thereby improve its pattern accuracy. This phase shift method is further divided into several methods, one of which, in particular, is known as Leverson method in which a difference in phase between adjacent two light permeable parts is alternately provided to be 180 degrees. In this method, however, it is difficult to exhibit the desired effects when three or more of patterns are provided adjacent to each other. That is, when a light phase difference between two patterns is set to be 180 degrees, one pattern becomes in phase with either one of the previous two patterns. As a result, disadvantageously, the patterns having a phase difference of 180 degrees can be solved but the patterns having a phase difference of 0 degrees cannot be solved. In order to eliminate this disadvantage, it becomes necessary to re-study the device design from its beginning and this involves considerable difficulties in putting it into practical use.
As another technique which is based on the phase shift method and eliminates the need for modifying the device design, the use of a half-tone phase shift mask having a light semi-transparent film as a phase reverse layer has been proposed. In order to maximize the merits of the phase shift method, it is important to optimize a phase difference between lights passed through a transparent part and a semi-transparent part as well as the amplitude transmissivity ratio of the transparent part and the semi-transparent part. The phase difference and amplitude transmissivity ratio are uniquely determined by the optical constants (complex refractive indexes n-ik: i being unit imaginary number) of these parts and the thicknesses of the films. In other words, in order to obtain a desired phase differences, and amplitude transmissivity ratio, it is necessary to satisfy the relationship between the optical constants and film thicknesses. However, since the optical constant is inherent or intrinsic in the film material, it is difficult to satisfy the relationship with a single layer film.
FIG. 45 shows a structure of an ideal half-tone phase shift film. A mask prepared according to this method comprises a light transmissive substrate 1 having a light transparent part 1a and a light semi-transparent part 1b formed on the substrate. The light semi-transparent part 1b is formed to have an amplitude transmissivity of 10 to 40% with respect to the light transparent part 1a, and at the same time the phase of light passing through the light semi-transparent part 1b is changed by 180 degrees with respect to the light permeable part 1a. To this end, the light semi-transparent film 1b is of a two-layer structure which comprises a first layer 2 for satisfying the above-mentioned purpose and a second layer 3 for adjusting a total of phase differences of the first and second layers to be 180 degrees.
In this way, in the prior art half-tone phase shift mask, the half-tone part has the two-layer structure, the amplitude transmissivity is adjusted by the first layer 2, and the phase difference is adjusted by the second layer 3 so that a total of the phase differences of the first and second layers becomes 180 degrees. And the first layer is made of Cr, MoSi or the like, while the second layer is made of SiO.sub.2, MgF.sub.2, CaF.sub.2, Al.sub.2 O.sub.3 or the like. Accordingly, in order to form the half-tone part, it is necessary to perform film formation in two different environments. For example, a sputtering system is used to form a Cr film as the first layer, while a chemical vapor deposition (CVD) process is used to form an SiO.sub.2 film as the second layer.
However, this sort of method is disadvantageous in that the film formation is carried out twice separately, which results in undesirable attachment of dust during transportation of the film and thus an increase in the number of defects in the resultant film. Further, this method has a processing problem that, since different elements must be used for the formation of the films, etching must be carried out with use of a plurality of different sorts of gases (for example, when it is desired to form Cr and SiO.sub.2 layers as the semi-permeable film, the Cr layer is processed with use of a Cl-series gas whereas the SiO.sub.2 layer is processed with use of a fluorine-series gas). Further, though the second layer is made of a transparent film, since the transparent film has a small refractive index, the thickness of the phase shifter film must be large. For this reason, its processing accuracy becomes bad.
In addition, for the purpose of preventing leakage of exposure light from an aligning or inspecting mark which is present on an area other than the pattern area on the mask in a light exposure process, a blind is provided to a projection aligner to cut off any light from the area other than the pattern area. In this connection, since the blind causes formation of a blur or dim image of about 100 .mu.m on the wafer, so that the blind cannot serve to define or mark off the pattern area on the wafer. For this reason, in the prior art, a light shielding pattern 101 is formed so as to surround the outer periphery of the pattern area on the mask, as shown in FIG. 46(a). However, when the exposure mask is made of only the semi-permeable film, such a semi-transparent pattern 201 is used in place of the light shielding pattern for marking off the pattern area which is present at the outer periphery of the pattern area, as shown in FIG. 46(b). In this case, the light passed through the semi-transparent film present at the boundary of the pattern area is irradiated on the wafer against adjacent patterns by an amount corresponding to the transmissivity of the semi-transparent film times the amount of exposure light. For this reason, this involves such a problem that, for the irradiation area, the amount of exposure light becomes substantially excessive so that the pattern area becomes too narrow and further its focal depth becomes insufficient, as shown in FIG. 47.
In this way, the prior art phase shift mask of the half-tone type has had such a problem that the formation of the semi-transparent film requires a number of necessary steps and also causes the attachment of dust to the film and generation of defects therein, which results in difficulty in exhibiting the maximum phase shift effects.
Further, when the exposure mask is made of only the semi-transparent film, the semi-transparent pattern is used even for the outer periphery of the pattern area which is intended to mark off the pattern area. Thus, the light passed through the semi-transparent film present at the boundary of the pattern area is irradiated on the wafer against an adjacent pattern by an amount corresponding to the transmissivity of the semi-transparent film times the amount of exposure light. For this reason, this involves such a problem that, for the irradiation area, the amount of exposure light becomes substantially excessive so that the pattern area becomes too narrow and further its focal depth becomes insufficient.
In the case of using a shifter edge type phase shift mask, a relatively large pattern is made of a light shielding film, while a fine pattern is made of a transparent film which has a light transmissivity of 100% and the phase of light passing through which is different by 180 degrees with respect to a substrate.
However, even the use of the shifter edge type phase shift mask or the half-tone type phase shift mask has had such a problem that, when a relatively large pattern is made of a light shielding film and a relatively small pattern is made of a phase shifter, that is, when the single mask comprises the light shielding film, semi-transparent film and transparent film, these films are formed conventionally through independent exposure processes, which results in that a relative position aligning accuracy between the films is reduced.
In addition, in this technique, it is difficult to exhibit its effects when three or more patterns are positioned adjacent to each other. In other words, when two patterns have a light phase difference of 180 degrees therebetween, one of the two patterns is in phase with either one of the previous two pattern. This results disadvantageously in that the patterns having a phase difference of 180 degrees can be solved but the patterns having a phase difference of 0 degrees cannot be solved. This advantage cannot be solved without re-studying the device design from its beginning, thus, it requires considerable difficulties to immediately put into its practical use.